This lesson explores the kinetic molecular theory and how it pertains to the properties of solids and liquids. You'll learn the properties of solids and liquids, discover the types of intermolecular attractions that occur between them and gain an understanding how phase changes happen.
Kinetic Molecular Theory
Take a glass of water. Drop a few drops of red food coloring in it. What happens? The red food coloring drops should make their way down the glass of water slowly, spread out and finally tint all of the water a reddish color. Why does this happen? It happens because both substances are made out of molecules that are constantly moving. These molecules have energy; one of the fundamental principles of the kinetic molecular theory.
The Kinetic Molecular Theory (KMT) is a model used to explain the behavior of matter. It is based on a series of postulates.
Some of the postulates of KMT are as follows:
- Matter is made of particles that are constantly in motion. This energy in motion is called kinetic energy.
- The amount of kinetic energy in a substance is related to its temperature.
- There is space between particles. The amount of space in between particles is related to the substance's state of matter.
- Phase changes happen when the temperature of the substance changes sufficiently.
- There are attractive forces in between particles called intermolecular forces. The strength of these forces increase as particles get closer together.
In this lesson, we will focus on how KMT can be used to explain the properties of liquids and solids.
KMT and Properties of Liquid
Check out these two pictures of liquid water:
Water in pool and at molecular level
One is a photo of water in a swimming pool; the other is of liquid water on the molecular level. What properties of liquids are evident in these two pictures?
One of the most notable properties of liquids is that they are fluid and they can flow. Liquids have definite volume, but not a definite shape. Liquids are said to have low compressibility; in other words, it's hard to pack liquid particles closer together. Compared to gases, there is relatively little space between particles. Compared to solids, however, liquids have some space between particles. This, in tandem with the fact that liquid particles also have relatively more energy than solid particles, is what allows liquids to flow. On the molecular level, these two factors give liquids the look of being disorganized.
The types of intermolecular forces in a liquid depend on the chemical make up of the liquid itself. Strength of intermolecular force is related to the type of intermolecular force, but it is also affected by the amount of kinetic energy in the substance. The more kinetic energy, the weaker the intermolecular forces. Liquids have more kinetic energy than solids, so the intermolecular forces between liquid particles tend to be weaker. We will discuss types of intermolecular forces later.
KMT and Solids
Now, let's check out some solids:
Pyrite crystals and phosphorus trioxide molecular structure
One of these images is of pyrite crystals in their naturally occurring cubic form; the other is the structure of phosphorus trioxide on the molecular scale. What properties of solids do you notice?
Solid substances have definite shapes and volumes. Solid particles do move, but not very far! Solid particles have relatively little kinetic energy and vibrate in place. Because of this, they can't flow like liquids. Most solids are arranged in a tightly packed crystalline structure. The crystalline structure is an orderly, repeating arrangement of particles called a crystal lattice. The shape of the crystal shows the arrangement of the particles in the solid.
Some solids aren't crystalline-shaped. The ones that aren't are called amorphous solids. Amorphous solids don't have orderly internal structures. Examples of amorphous solids include rubber, plastic and glass. Wax is also an amorphous solid. It can be molded into any shape and remolded anytime it is warmed up a bit.
As previously noted, intermolecular forces are the attractive forces between particles. They are distinctly different from the bonds that occur within particles. The type of intermolecular forces present depends on the type of particles present.
Hydrogen bonds occur between polar molecules that contain an oxygen, nitrogen or fluorine atom covalently bonded to a hydrogen atom. The intermolecular attraction happens between the partially negatively charged oxygen, fluorine or nitrogen and the partially positively charged hydrogen of a neighboring molecule. Hydrogen bonds are relatively strong intermolecular forces.
Ion-dipole forces occur between an ion and a polar molecule. An ion will form an attraction with an oppositely charged pole of a neighboring molecule. Salt water is loaded with ion-dipole attractions. Positive sodium ions are attracted to the negative poles of water molecules. Negatively charged chloride ions are attracted to the positive poles of water molecules.
Dipole-dipole forces occur between the oppositely charged poles of polar molecules.
Dipole-induced dipole forces occur when a polar molecule induces a temporary dipole moment in a neighboring non-polar molecule. You might be asking yourself, what? How can a non-polar molecule get a dipole moment? The negatively charged electrons that buzz around an atom or molecule are mobile and polarizable. If a negatively charged particle approaches a bunch of electrons, they will be repelled by it and move as far away as possible. Now, imagine the negative pole of a molecule approaches a non-polar molecule. The electrons move to the side facing away from the incoming negative pole. We now have one side that is more negative and one side that is more positive: it's an induced dipole! These intermolecular forces are relatively weak.
London Dispersion Forces (LDF) are the weakest type of intermolecular force. LDFs occur between non-polar molecules when the random movement of electrons around a molecule creates a temporary dipole that induces a temporary dipole in a neighboring molecule.
The amount of kinetic energy in a substance is related to its phase. Gases have more kinetic energy than liquids. Liquids have more kinetic energy than solids. When a substance increases in temperature, heat is being added, and its particles are gaining kinetic energy.
Because of their close proximity to one another, liquid and solid particles experience intermolecular forces. These forces keep particles close together. The more kinetic energy particles have, the weaker these forces become. At certain temperatures, particles overcome enough intermolecular forces to experience a significant change in properties. At this point, a phase change occurs. There are specific temperatures for every substance that coincide with the amount of kinetic energy necessary for particles to have properties characteristic of liquid, solid or gas.
The Kinetic Molecular Theory (KMT) is a model used to explain the behavior of matter.
Properties of liquids include:
- Definite volume but indefinite shape
- Relatively incompressible
- Particles have more energy than solids, but less energy than gases
- Liquids can flow
Properties of solids include:
- Definite shape and volume
- Very incompressible
- Particles have relatively little energy
- Solid particles vibrate in place
- Solids often have crystalline structures
Intermolecular Forces (IMFs) are the attractive forces between particles. The type of IMF present depends on the chemical composition of the substance.
Phase changes occur when particles experience a significant change in temperature. Temperature measures the amount of kinetic energy of a substance. A substance's kinetic energy is related to its phase.
Once you've finished with this lesson, you will be able to:
- Describe the Kinetic Molecular Theory
- Identify the properties of solids and gases
- Recall the types of intermolecular forces
- Explain how phase changes occur